ci: minor fixes

[DaniPopes]

• actions/checkout: depth -> fetch-depth

Unexpected input(s) 'depth', valid inputs are ['repository', 'ref', 'token', 'ssh-key', 'ssh-known-hosts', 'ssh-strict', 'persist-credentials', 'path', 'clean', 'sparse-checkout', 'sparse-checkout-cone-mode', 'fetch-depth', 'fetch-tags', 'lfs', 'submodules', 'set-safe-directory', 'github-server-url']

• rust-cache: add with: cache-on-failure: true
• Unpin most Ubuntu runners: as we know from Foundry, we only need to pin to Ubuntu-20.04 when building release binaries which users will consume, since otherwise they'll get libc errors.